Field of the Invention
The invention relates to an active sensor element and to a method of determining the temperature of an active sensor element.
The power generated by an internal combustion engine depends on the air mass and the corresponding quantity of fuel which can be provided to the engine for combustion purposes. In order to increase the power of the internal combustion engine, it is necessary to supply a greater quantity of combustion air and fuel to the internal combustion engine. In the case of a naturally aspirated engine, this increase in power is achieved by increasing the cubic capacity or by increasing the rotation speed. However, increasing the cubic capacity leads, in principle, to heavier, larger and therefore more expensive internal combustion engines.
One commonly used technical solution to increasing the power of an internal combustion engine is supercharging. This term describes the precompression of the combustion air by an exhaust gas turbocharger or else by a compressor which is mechanically driven by the engine. An exhaust gas turbocharger essentially comprises a compressor and a turbine which are connected to a common shaft and rotate at the same rotation speed. The turbine converts the otherwise uselessly stored energy of the exhaust gas into rotation energy and drives the compressor. The compressor draws in fresh air, compresses it, and delivers the compressed air to the individual cylinders of the engine. An increased quantity of fuel can be supplied to the greater quantity of air in the cylinders, as a result of which the internal combustion engine experiences a considerable increase in power.
The combustion process is also influenced in a favorable manner, and so the internal combustion engine achieves a better overall degree of efficiency. Furthermore, the torque profile of an internal combustion engine which is supercharged using a turbocharger can be configured extremely favorably. The existing naturally aspirated engines in series production by vehicle manufacturers can be substantially optimized by using an exhaust gas turbocharger without great intervention in the design of the internal combustion engine. Supercharged internal combustion engines generally have a lower specific fuel consumption and emit fewer pollutants. Furthermore, turbocharged engines are quieter than naturally aspirated engines of the same power since the exhaust gas turbocharger itself acts as an additional silencer.
In internal combustion engines with a large operating rotation speed range, for example in internal combustion engines for passenger cars, a high charge pressure is required even at low engine speeds. For this reason a charge pressure control valve, what is known as a waste-gate valve, is employed in these turbochargers (the terms turbocharger and exhaust gas turbocharger are used synonymously here). The selection of a corresponding turbine casing means a high charge pressure is quickly built up even at low engine speeds. The charge pressure control valve (waste-gate valve) then limits the charge pressure to a constant value as the engine speed increases. Turbochargers with variable turbine geometry (VTG) are used as an alternative to this. In these turbochargers, the charge pressure is regulated by means of the variation in the turbine geometry.
As the quantity of exhaust gas increases, the maximum permissible rotation speed of the combination comprising the turbine wheel, the compressor wheel and the turboshaft, which combination is also called the running gear of the turbocharger, can be exceeded. In the event of the rotation speed of the running gear being exceeded to an impermissible extent, said running gear would be destroyed, which is the same as total destruction of the turbocharger. Even modern and small turbochargers with considerably smaller turbine and compressor wheel diameters which exhibit improved rotation acceleration behavior on account of a considerably smaller moment of mass inertia are affected by the problem of the permissible maximum rotation speed being exceeded. Depending on the configuration of the turbocharger, even an instance of the rotation speed limit being exceeded by approximately 5% leads to complete destruction of the turbocharger.
Charge pressure control valves which are actuated by a rotation speed sensor according to the prior art have proven useful for limiting the rotation speed. If the charge pressure exceeds a predefined threshold value, the charge pressure control valve opens and conducts a portion of the exhaust gas mass flow past the turbine. This consumes less power on account of the reduced mass flow rate, and the compressor power drops to the same extent. The charge pressure and the rotation speed of the turbine wheel and of the compressor wheel are reduced. WO 2006/005662 A1 discloses the use of active sensors, for example Hall sensors or magnetoresistive sensors, for measuring the rotation speed of the turboshaft and for actuating the charge pressure control valve. However, the temperature prevailing in and across the turbocharger is a critical variable for these active sensors. Sensors produced on the basis of a semiconductor can be used at temperatures of up to 170° C. If the temperature in the semiconductor rises above this value, it may lead to irreparable damage to the sensor, as a result of which a faulty rotation speed signal may be produced in turn, this ultimately possibly leading to excessive rotation of the running gear, as a result of which the turbocharger would be destroyed.